1. Field of the Invention
The present invention relates to a rod lens fixing method for fixing a rod lens in a sleeve to be used for an optical semiconductor module or the like by means of soldering and laser welding.
2. Description of the Prior Art
In an optical communication system, for example, employing an optical fiber as an optical transmission line, an optical semiconductor module is used for the purpose of introducing an outgoing light from an optical semiconductor device such as a laser diode (LD) and a light emitting diode (LED) into the optical fiber. In the optical semiconductor module, the optical semiconductor device and an incident end surface of the optical fiber are fixed in a predetrmined positional relation, and a condensing lens is provided therebetween. In this kind of optical semiconductor module, a relative positional relation between components has a direct influence upon an optical coupling efficiency. Accordingly, it is required to position each component with a high accuracy, e.g., 1 .mu.m or less. Moreover, it is also required to maintain such a positioning accuracy for a long period of time.
FIG. 1A is an elevational view of a lens assembly manufactured by a lens fixing method in the prior art, and FIG. 1B is a vertical sectional view of the lens assembly shown in FIG. 1A. Reference numeral 2 denotes a flanged split sleeve formed at its own end with a flange 4. The flanged split sleeve is formed of stainless steel or the like. A condensing rod lens 6 is press-fitted in the flanged split sleeve 2, and is fixed by solders 8 and 9 to the flanged split sleeve 2 at a portion of an axial slit 5 and opposite ends of the flanged split sleeve 2. To enable the rod lens 6 and the flanged split sleeve 2 to be welded together, an outer circumference of the rod lens 6 and a predetermined portion of the flanged split sleeve 2 are plated with gold. Thus, a lens assembly 10 is constructed. As shown in FIG. 2B, the lens assembly 10 is inserted into a stepped sleeve 12 from one end thereof, and as shown in FIG. 2A, the flange 4 is laser-welded to one end surface of the stepped sleeve 12 at four points P. On the other hand, a ferrule 14 connected to an optical fiber 16 is inserted into the stepped sleeve 12 from the other end thereof. After a relative positional relation between the rod lens 6 and the ferrule 14 in the stepped sleeve 12 is adjusted, the ferrule 14 is similarly laser-welded to the stepped sleeve 12. In this way, a fiber collimator (virtual fiber assembly) 18 is formed.
After the lens assembly 10 and the ferrule 14 are laser-welded to the stepped sleeve 12, annealing of the fiber collimator 18 is usually repeated for a long period of time, so as to remove stresses due to the laser welding. During this annealing, tensile forces act in the flange 4 of the flanged split sleeve 2 to expand the flange 4. As a result, the rod lens 6 is drawn in radially outward directions, causing the generation of cracks in the rod lens 6 as shown by arrows C in FIG. 2A from near the slit 5 of the flange 4.